This invention relates to a method for preparing an alkaline corrugating adhesive. More particularly, this invention is directed to a method for preparation of an alkaline starch corrugating adhesive by continuous pressure cooking which adhesive has good tack properties.
The procedures employed in the production of corrugated board normally involve a continuous process wherein a strip of paperboard is first corrugated by means of heated, fluted rolls. An adhesive is applied to the protruding tips on one side of this corrugated strip, and a flat sheet of paperboard, referred to as a liner, is then brought into contact with these tips and, by application of heat and pressure, a bond is subsequently formed. The product thus prepared is known as a single-faced board inasmuch as it has a liner on only one surface thereof. However, by repeating the above-described bonding process on the exposed side of the corrugated strip of the single-faced board, there is then produced the so-called double-faced board comprising an inner corrugated layer sandwiched between two liners.
The particular adhesive employed in the corrugating process is selected on the basis of several factors, including the type of bond required in the final application of the finished corrugated product. Starch-based adhesives are most commonly used due to their desirable adhesive properties, low cost and ready availability.
The most fundamental of starch corrugating adhesives is that adhesive produced by gelatinizing starch in water with caustic soda to yield a primary mix of gelatinized carrier, which is then slowly added to a secondary mix of raw (ungelatinized) starch, borax, and water to produce the full-formulation adhesive, which is alkaline in character. In the corrugating process the adhesive is applied to the tips of the fluted paper medium, whereupon the heat causes the raw starch to swell and absorb water from the carrier starch. There are different theories regarding the respective roles of the raw starch and the carrier in the development of the adhesive properties, but there is substantial evidence to support the view that the carrier contributes to the bond strength and set speed of the adhesive, and that good tack in the carrier leads to better green bond strength (tack), and therefore improved runnability, in the full-formulation adhesive (see R. Williams, C. Leake, and M. Silano, TAPPI, Vol. 60, Nr. 4, April 1977, pp. 86-89).
Conventional batch methods are normally employed to cook carrier starches for use in corrugating adhesives. In such a procedure the starch is slurried in the cook tank, heated, e.g., to about 74.degree. C., and held for ten minutes at that temperature. Batch size should be minimized to prevent overcooking and mechanical breakdown on holding.
Recently, continuous pressure cookers, commonly called jet cookers, have been developed to permit rapid cooking of starch, normally at temperatures of about 180.degree.-360.degree. F. (82.degree.-182.degree. C.), for use in various applications such as wet-end addition to paper, sizing, textile finishing and food stabilization. In the typical jet-cooking process, dry starch is slurried in water and pumped through a very small orifice to a cooking chamber (valve) of the cooking apparatus where it is immediately contacted by live steam such that intimate mixing occurs. In some installations water is provided to dilute the cold slurry prior to or after cooking. The heat of the steam gelatinizes the starch, and the mechanical shearing action of the expanding steam disrupts the swollen granules, all taking place in a matter of seconds. Advantages of continuous (jet) cooking over conventional batch cooking are that cooking conditions may be changed easily and promptly and that storage of cooked paste is minimal. The system enables the selection of a temperature and flow rate which will give the desired degree of swelling or dispersion.
Gelatinized carrier starches having good water resistance and tack are generally not prepared by continuous pressure cooking because the cooked starches which impart good water-resistance properties (i.e., unmodified and acid-converted high-amylose starches) retrograde during the operation, thereby preventing proper flow of the cooked starch into the raw portion of the adhesive, with consequent poor tack performance of the adhesive.
Accordingly, it is an object of the present invention to provide a method for preparing an alkaline starch corrugating adhesive with good tack from a high-amylose starch in a continuous pressure cooker.